1. Field of the Invention
The present invention relates to an image forming apparatus such as a color copier or color laser beam printer.
2. Description of the Related Art
The use of color in image forming apparatuses, such as copiers or laser beam printers, is advancing rapidly. In a color copier, a photosensitive drum as an image carrier and a transferring drum as an image forming medium supporting member which supports the image forming medium are rotated in synchronous contact. A light beam corresponding to each of the original color images is scanned over the image carrier by means of a scanning device. Thus, a latent image corresponding to each of the original color images is formed on the image carrier. This latent image formed on the image carrier is developed as color toner images by multiple colored developing agents each corresponding to the original color image. Each of these respective color toner images which has been developed is transferred in succession and superimposed one on another on the image forming medium supported on the transferring drum. By this means a toner image corresponding to the original color image is formed and fixed on the image forming medium.
In this type of color image forming apparatus, the color image is formed by the successive superimposition of each of the color toner images on the image forming medium. Therefore, the most important factor is to prevent color slippage.
The image forming apparatuses, in the prior art are designed to equalize the distance from the exposure position on the image carrier to the transferring position in the direction of rotation and the distance from the leading edge position of the image forming medium to the transferring position in the direction of rotation. Therefore, the formation of the latent image corresponding to each of the colors of the original image is commenced when the leading edge position of the image forming medium on the transferring drum is detected. By this means, the matching between the toner images of each colors is performed.
Furthermore, as one method of preventing color slippage, a control method which concerns the drive of the image carrier and the image forming medium supporting member is disclosed in, for instance, Japanese Laid-Open Patent No. 61-217076. In the control method disclosed in that Patent, a driving mechanism is provided which does not give rise to pitch randomness and color randomness. The image carrier and the image forming medium support member are designed to be driven independently by separate driving devices. Thus the load fluctuations of the image carrier are not transmitted to the image forming medium support member, nor are the load fluctuations of the image forming medium support member transmitted to the image carrier.
However, in this type of image forming apparatus, when executing multiple transferring for each color, the leading edge positions of the toner image on the image carrier and of the image forming medium are matched. Then, when executing the image forming process by means of leading edge position matching between the respective color toner images, the rotational drive of the image carrier and the rotational drive of the transferring drum on which the image forming medium is supported and, moreover, the drive of a polygon mirror as the scanning device which scans the light beam over the image carrier are driven asynchronously. Therefore, when phase slippage occurs between the detection signals which detect the leading edge position of the image forming medium and the horizontal synchronization signals for controlling the image forming process, this phase slippage will accumulate during the execution of the image forming processes for multiple colors. The control of the leading edge position matching of the image forming medium will be affected by this. This results in the problem of color slippage occurring in the image on the image forming medium when the images formed on the image carrier are transferred on the image forming medium.
That is, in a prior art image forming apparatus, when executing multiple transferring for each color, the rotational drive of the image carrier and the rotational drive of the tranferring drum and, moreover, the drive of the scanning device which scans the light beam over the image carrier are driven asynchronously. Therefore, phase slippage occurs between the detection signals which detect the leading edge position of the image forming medium and the horizontal synchronization signals for controlling the image forming process. The control of the leading edge position matching of the image forming medium is affected by this. This results in the problem of color slippage occurring in the image on the image forming medium.